Insight into the aggregation of coal derived material in solvent — an observation by size exclusion chromatography

The aggregation of coal derived material in solvents was investigated by size exclusion chromatography (SEC), using a polystyrene–divinylbenzene packed column and eluting with NMP, NMP/CS2 (1:1, v/v) and NMP containing anion additives. The SEC chromatogram of coal extract using NMP as the eluent shows two main peaks. One appears near the exclusion limit of the column (shorter retention time) and the other one at the longer retention time. Changing the eluent to NMP/CS2 (1:1, v/v) mixed solvent results in the disappearance of the material eluted near the exclusion limit. Instead, all the fractions appear at longer retention time. Adding anion additives (5–10 mM), e.g. tetrabutylammonium acetate and NMP 1,1,2,3,3-pentacyanopropenide, into NMP eluent also results in the disappearance of the material eluted near the exclusion limit. The observed changes are thought to be due to the disruption of coal molecular aggregation rather than changes in solvent properties. The material eluted near the exclusion limit is strongly aggregated, its size depends on the extent of aggregation and the solvent used. Disruption of the aggregated coal molecule was responsible for the changes in the SEC chromatogram. In addition, the aggregation of the coal extract was found to increase with decreasing coal rank. For the different extract fractions, the aggregation increases in the order: acetone soluble, pyridine soluble–acetone insoluble and pyridine insoluble–NMP/CS2 (1:1, v/v) mixed solvent soluble.